Demand continues to increase for four-wheel drive vehicles based on the enhanced on-road and off-road traction control they provide. In many four-wheel drive vehicles, a transfer case is installed in the drivetrain and is normally operable to deliver drive torque to a primary driveline for establishing a two-wheel drive mode. The transfer case may be further equipped with a clutch assembly that can be selectively or automatically actuated to transfer drive torque to the secondary driveline for establishing a four-wheel drive mode. These “mode” clutch assemblies may include a simple dog clutch that is operable for mechanically shifting between the two-wheel drive mode and a “locked” (i.e., part-time) four-wheel drive mode, a sophisticated automatically-actuated multi-plate clutch for providing an “on-demand” four-wheel drive mode, or some other variant.
On-demand four-wheel drive systems are able to provide enhanced traction and stability control and improved operator convenience since the drive torque is transferred to the secondary driveline automatically. An example of a passively-controlled on-demand transfer case is shown in U.S. Pat. No. 5,704,863 where the amount of drive torque transferred through a pump-actuated clutch pack is regulated as a function of the interaxle speed differential. In contrast, actively-controlled on-demand transfer cases include a clutch actuator that is adaptively controlled by an electronic control unit in response to instantaneous vehicular operating characteristics detected by a plurality of vehicle sensors. U.S. Pat. Nos. 4,874,056, 5,363,938 and 5,407,024 disclose various examples of adaptive on-demand four-wheel drive systems.
Due to the cost and complexity associated with such actively-controlled, on-demand clutch control systems, recent efforts have been directed to constructing simplified transfer cases that provide dedicated operating modes without incorporating multiple friction plate clutches and the associated actuators. It may be beneficial to continue to develop transfer cases that do not require large electric actuator motors or hydraulic pumps but instead take advantage of the kinetic energy of the vehicle to complete a shift. In addition, it may be desirable to disconnect various power transfer components from the load path during certain modes of operation to increase the fuel efficiency of the vehicle.